1. Field of the Invention
The present invention generally relates to an automatic document feeder (ADF) and an image forming apparatus including the ADF, and, more particularly, to an ADF that automatically conveys original document medium (sheets) one sheet by one sheet from a stack of the original document placed on a document tray, and an image forming apparatus, such as a facsimile, a copier, or a multifunction peripheral, including the ADF.
2. Description of the Related Art
This type of ADF includes, as a driving source, an alternating-current (AC) motor that runs on alternating current or a direct-current (DC) motor that runs on direct current. DC motors consume less electric power than AC motors. Accordingly, ADFs that use DC motors rather than AC motors as driving source to reduce power consumption have become widespread.
DC motors differ from AC motors in that, when placed in a runaway state in which a motor rotates in an uncontrolled manner against a control command, loss of synchronization, which is slowdown or stopping of the motor resulting from a difference between a synchronization speed and an actual rotation speed, is less likely occur. For this reason, a countermeasure against runaway is generally taken for a DC motor.
Examples of the countermeasure include providing a switch such as an interlock switch or an interrupter in an ADF and interrupting power supply to a DC motor when occurrence of runaway is predicted.
In an ADF, paper jam can occur because an original document being conveyed is jammed into a component such as a conveying roller due to a type, such as size, a thickness, or a material, of the document, a bent or the like state of the document, or because the document is stapled. When jam occurs, a user generally performs what is referred to as jam recovery by opening a sheet feed cover of the ADF and removing a jammed document. An ADF typically includes an open/close detection sensor that detects whether the sheet feed cover is open or closed and is configured to prevent the ADF from operating during the jam recovery by interrupting power supply to a driving source of the ADF when the open/close detection sensor detects that the sheet feed cover is open. In some ADFs, the interlock switch described above combines a function of performing open/close detection of the sheet feed cover and a function of switching on and off the power supply. Such an interlock switch allows a user to perform jam recovery safely by, when the sheet feed cover of the ADF is opened by the user, interrupting power supply at the same instant when the sheet feed cover is detected as being open.
However, such an interlock switch that combines the two functions is disadvantageous in that a motion produced by a user when pulling out a jammed document rotates a conveying roller, which in turn rotates a motor.
When the motor is rotated, the rotation induces an electromotive force in the motor, causing the motor to function as a generator. As a result, a voltage in a detection circuit that detects whether the sheet feed cover is open or closed can rise to reach a detection voltage. This can lead to false detection that the sheet feed cover is detected as being closed even though the sheet feed cover is open.
Japanese Patent Application Laid-open No. 2003-323086 discloses an example of a conventional image forming apparatus including an ADF that prevents this type of false detection. The image forming apparatus includes a control unit that requires at least two power sources that are independent from one another, a driving unit that drives a load according to a command fed from the control unit, an interlock switch arranged on a driving power-source line through which driving power source is supplied from the control unit to the driving unit, and a backflow preventer diode arranged on a control power-source line through which control power source is supplied from the control unit to the driving unit.
This ADF is configured in such a manner that, when a user opens a door (a sheet feed cover) to perform jam recovery to remove jammed paper or the like, the interlock switch is switched off, and power supply is automatically turned off. The ADF is also configured in such a manner that, when the door is closed, the interlock switch is switched on, and the power supply is automatically turning on. When the power supply is turned on, a transient large current, which is generally referred to as an inrush current, back flows into the control power-source line, which can be a cause of malfunction of the interlock switch.
However, in this ADF, the backflow preventer diode provided on the control power-source line blocks backflow of the inrush current; accordingly, malfunction in on-off switching of the interlock switch is prevented.
Although the conventional ADF described above is configured to solve the malfunction of the interlock switch resulting from the inrush current, the ADF is not configured to solve a problem of malfunction of the interlock switch that combines the function of performing open/close detection of the sheet feed cover and the function of switching on and off the power supply. Accordingly, a challenge of preventing malfunction of the interlock switch that combines the two function is not solved yet, which is a problem.
Whereas the conventional ADF can prevent malfunction in on-off switching of the interlock switch by blocking the backflow of the inrush current, the conventional ADF is insufficiently capable of solving the malfunction in on/off detection of the sheet feed cover described above. Specifically, when a motor is rotated during jam recovery, a voltage in the detection circuit that detects open/close of the sheet feed cover can rise to reach a detection voltage as already described above. Thus, there is a problem that the challenge of preventing false detection of the interlock switch during jam recovery is not solved yet.
In view of the problem, there exists a need for providing an ADF capable of preventing an interlock switch, which combines a function of switching on and off power supply to a driving component and a function of performing open/close detection of a sheet feed cover, from making false detection in open/close detection of the sheet feed cover.